Ethylene/.alpha.,.beta.-unsaturated carboxylic acid interpolymers are thermoplastic polymers which are useful as adhesives. An adhesive layer or coating of the interpolymers can be applied to a substrate, such as paper or metal foil, by extrusion coating or other conventional techniques. The coating thickness, however, cannot be reduced much beyond about 0.4 mils, or about 6 pounds per ream, due primarily to the melt strength of the interpolymer. The line speed and the die/system design also affect the ability of the interpolymer to coat the substrate at minimal thicknesses. Generally, such coatings are thicker than necessary to provide the desired properties in a laminate structure. Thinner coatings would be preferred from an economic standpoint.
Thin coatings have been applied to substrates using a gravure or meyer rod technique and an aqueous dispersion of the polymeric adhesive, but the dispersions have been only available at low solids concentrations.
Many methods of forming dispersions from ethylene/.alpha.,.beta.-unsaturated carboxylic acid interpolymers are disclosed in the literature. One method involves neutralizing at least portion of the carboxylic acid groups with an alkaline base to form a dispersible carboxylic acid salt. The dispersions formed using this technique, especially when the .alpha.,.beta.-unsaturated carboxylic acid content is low or the molecular weight of the interpolymer is high, typically have a high content of non-dispersibles, i.e., greater than 1%. When such dispersions are coated onto a substrate, the coating tends to be uneven and variable and, in addition, can contaminate the equipment due to the high percentage of non-dispersibles. The non-dispersibles gum up the equipment and cause equipment shutdown and subsequent cleanup. If more base is utilized to reduce the non-dispersibles content, the percentage of dispersed solids decreases even further and makes the overall process uneconomical. Dispersions of interpolymers having low carboxylic acid content (e.g., less than about 15% acid content) are particularly difficult to form.
International Patent Application Publication Number WO 89/07519 (Patton et al.) discloses a number of techniques for forming dispersions from ethylene/.alpha.,.beta.-unsaturated carboxylic acid interpolymers, including using a combination of alkali metal hydroxide and ammonia solution. Patton et al. did not recognize, nor do they suggest, using a mixture of bases in the proportions set forth below.